Reassessing the Cutoff Level for Poliovirus Antibody in Patients after Successful Chemotherapy for Leukemia or Hematopoietic Stem Cell Transplantation

To the Editor—The serologic profile of poliovirus neutralizing antibody in patients with acute leukemia prior to vaccination against poliovirus and after treatment for leukemia revealed that almost 11% of patients had protective antibody to all 3 poliovirus serotypes. After 12 months of vaccination, only 47% of patients had protective antibody to all 3 po-liovirus serotypes. The antibody titer cutoff used to label disease protection was у1:8 [1]. Identical susceptibility in children after hematopoietic stem cell trans-plantation before and after vaccination was 29% and 92%, respectively [2]. With the imminent global control of polio, the antibody titer cutoff level in patients after completion of successful chemotherapy for leukemia or after hematopoietic stem cell transplantation would be worth scrutiny. The prevailing у1:8 titer might be responsible for false confidence. The existing cutoff level of у1:8 has been known to be protective against wild or Sabin attenuated poliovirus strains. This titer might not be adequate against any budding vaccine-derived poliovirus isolates. Recently, type 2 and type 3 evolving or highly divergent vaccine-derived poliovirus isolates were isolated from sewage in Israel. Neutralization data on these isolates demonstrated viral genetic diversity and antigenic divergence. During their neutralization, there was an average 3.3-fold decrease in geometric mean titer, even though the protective antibody titers for Sabin and wild strains exceeded у1:8. Moreover, 10 (7%) of 150 individuals aged 20–50 years had titers below the minimum protective level of у1:8 against у1 vaccine derived poliovirus strain [3]. Prospective studies to work out the use of simpler vaccination schedules for any earlier vaccinations after successful che-motherapy or transplantations [1, 2] should aim to express polio antibody quantum in international units, rather than use an arbitrary dilution figure. Prospective comparison of inter-and intral-aboratory serologic data would be better illustrated with the use of international units. The antibody content would be more explicit if expressed in international units, rather than using an arbitrary dilution figure. Serologic data on representative post-OPV serum samples from Ger-many were expressed in such units [4].